1. Field of the Invention
The present invention relates to patterning of a foil surface. More particularly, the present invention relates to etching an aluminum foil for use in electrolytic capacitors.
2. Related Art
Electrolytic capacitors are compact, high voltage energy storage means used in many applications, including implantable medical devices. For example, Implantable Cardioverter Defibrillators (ICDs) conventionally include electrolytic capacitors because such capacitors have a high energy density and can withstand a relatively high voltage, ICDs typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an ICD can utilize two 250 to 500 volt electrolytic capacitors in series to achieve a voltage of 500 to 1,000 volts.
Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween.
These capacitors must typically store approximately 10-100 joules. Because the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. Thus, their size can be relatively large, and it can be difficult to package them in a small implantable device. Currently available ICDs are relatively large devices (over 20 to 40 cubic centimeters (cc)), generally about 12-16 millimeters (mm) thick. A patient who has a device implanted can often be bothered by the presence of the large object in his or her pectoral region. For the comfort of the patient, it is desirable to make smaller ICDs. The size and configuration of the capacitors contribute 9 to 12 cc of the ICD volume.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. By electrolytically etching aluminum foils, an enlargement of a surface area of the foil will occur without enlargement of the overall capacitor. As a result of this enlargement of the surface area, electrolytic capacitors can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface. Likewise, etched-foil capacitors of a given volume can obtain a higher capacitance compared to unetched-foil capacitors.
Etching the foil increases the surface area of the foil by roughening an otherwise flat surface. A metal foil can be etched according to any method that increases the surface area, such as electrochemical etching, roughening the foil surface mechanically and chemical etching. Electrochemical etching increases the surface area of the foil by electrochemically removing portions of the foil to create etch tunnels. Electrochemical etching is done according to any known etch process, such as the ones discussed in U.S. Pat. No. 4,474,657 to Arora; U.S. Pat. No. 4,518,471 to Arora; U.S. Pat. No. 4,525,249 to Arora and U.S. Pat. No. 5,715,133 to Harrington et al., each of which is incorporated herein by reference in its entirety.
In a conventional electrolytic etching process, surface area of the foil is increased by removing portions of the aluminum foil to create etch tunnels. The foil used for such etching is typically an etchable aluminum strip of high cubicity. The etch initiation and hence the gain or capacitance of the foil is the result of several variables, such as foil cubicity, thermal oxide on the foil, and the electrochemical reaction.
As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance.
Creating a pattern on an aluminum foil surface has been previously demonstrated as a means to successfully increase surface area. For example, U.S. Pat. No. 7,150,767 to Schneider, et al., which is incorporated herein by reference in its entirety, discloses an etching process which applies a holographic image to a photoresist coated on a foil to create a pattern of photoresist on the foil prior to etching. The photoresist pattern on the foil surface allows for positional control of tunnel initiation. Alternatively, U.S. Pat. No. 6,224,738 to Sudduth, et al. and U.S. Pat. No. 6,736,956 to Hemphill et al., which are incorporated herein by reference in their entirety, disclose etching processes which utilize masking to control tunnel initiation.
By controlling the position of tunnel initiation, foils are etched more uniformly and have optimum tunnel distributions. The difficulty arises, however, in attempting to control the pattern on a 0.1 to 5 micron (μm) scale. Typical etching processes used to initiate tunnel formation can undercut a patterned resist. Tunneling can begin in directions other than orthogonal to the foil surface. This can cause release of the photoresist, increase the brittleness of the foil, and/or reduce the optimization of the morphology of the foil surface. There is a need therefore for a process of foil etching for use in electrolytic capacitors which allows for improved control of tunnel initiation on a micron or sub-micron level.